Tests for GimpGradientFile

 * Tests for GimpGradientFile.
 * Created two sample gradient files:
  * GIMP <= 1.2 gradient files don't contain a name metadata field and passed without change.
  * GIMP >= 1.3 gradient files do contain a name metadata field and failed, so this includes a fix.
 * Helps towards: Test coverage >= 80% #722.
 * flake8.